The light scattering based method for rapid detection and quantitative characterization of reversible macromolecular associations first reported in last year?s annual report has been extended to enable the detection and characterization of associations between two proteins in solution. We have demonstrated the capability of the technique to characterize multiple complexes of two proteins, and to characterize both self-association and hetero-association occurring in the same solution. Finally, the technique has been used to characterize the concentration-dependent distribution of multiple species in a solution of a single protein undergoing self-association to form linear oligomers of indefinite length. In collaboration with Peter McPhie (NIDDK), the effects of macromolecular crowding upon the stability of apomyoglobin with respect to denaturation by both heat and cold have been studied via circular dichroism spectroscopy. At pH 2, apomyoglobin is almost completely denatured, but in the presence of trichloroacetate at millimolar concentrations, adopts a compact non-native structure termed ?molten globule?, which unfolds at low temperatures (above freezing) as well as at high temperatures. In the presence of sufficiently high concentrations of a nonionic polysaccharide (dextran T10) the temperature for 50% heat unfolding is elevated and that for 50% cold unfolding is depressed, in accordance with earlier predictions of excluded volume theory. The effect of both trichloroacetate and dextran upon the temperature-dependent ellipticity of apomyoglobin may be accounted for semiquantitatively by a two-state model.